The present invention relates in general to materials handling vehicles and, more particularly, to methods and apparatus for braking such vehicles. While the present invention is generally applicable to materials handling vehicles, it will be described herein with reference to a rider reach fork lift truck for which it is particularly applicable and for which it is initially being utilized.
Braking of fork lift trucks may be accomplished through the use of a mechanical brake, such as disclosed in U.S. Pat. No. 5,057,728, which is incorporated herein by reference, or by using the dynamic braking characteristics of the electric traction motor, such as disclosed in U.S. Pat. No. 5,340,202, which is incorporated herein by reference.
The brake shown in the '728 patent is a three-step brake; that is, the brake is capable of providing three levels of braking force, depending upon braking requirements. Of course, mechanical brakes are subject to wear; therefore, to reduce wear, and since regenerative braking using the traction motor conserves battery charge, it is desirable to use the electric motor for as much of the braking requirements as possible. On a rider reach fork lift truck, a single, electrically powered and steerable drive wheel is mounted on one corner of the power unit of the truck, and a caster wheel is mounted on the other corner. For some trucks, the caster wheel is also provided with a brake or a caster brake.
Thus, while motor braking and mechanical braking of the drive wheel and, in some applications also mechanical braking of the caster wheel, are known for materials handling vehicles, there is an ongoing need to improve the methods and apparatus which operate and coordinate these braking systems for improved braking performance. Such improvements would better adapt braking performance for known operating conditions of travel direction and allowable travel speed/fork height, distribute braking requirements between mechanical braking and regenerative braking associated with the steerable drive wheel, use regenerative braking as much as possible, and engage the brake on the caster wheel in proper proportion to the brake on the drive wheel. Preferably, the improved braking control would be computer controlled and hence be easily adaptable in the field and also enable simplified manufacturing of materials handling vehicles utilizing the improved braking control.